Diamond has been employed as a semiconductor material for some time, and research into its practical application as a basis for semiconductor devices is ongoing.
In diamond, with its extraordinarily large bandgap of 5.5 eV, no intrinsic gap—in which carrier conduction is forbidden—as a semiconductor exists at temperatures of 1400° C. or less. And since the dielectric constant of diamond is a small 5.7, its breakdown electric field is a large 5×106 V·cm−1. Moreover, the carrier mobility for electrons/protons alike is a high 2000 cm2/V·s. Diamond also has an unusual property in that its electron affinity is negative. Having electrical characteristics such as these, diamond can be expected to produce such semiconductor devices as power devices that can withstand being operated at high frequency and high output power under high temperatures, light-emitting devices that enable the emission of ultraviolet rays, as well as electron-discharging devices that can be driven at low voltage.
In order to employ diamond as a material for semiconductor devices, the form of its electrical conductivity, p-type or n-type, must be controlled. By adding boron as an impurity into diamond crystal, p-type semiconductor diamond can be obtained. Diamond exists in nature as a p-type semiconductor, and p-type semiconductor diamond can by synthesized relatively easily by introducing gas containing boron atoms into a source-material gas using a chemical vapor synthesis (a CVD) method.
On the other hand, n-type semiconductor diamond does not exist in nature, and although its synthesis has until recently proven elusive, lately single-crystal n-type semiconductor diamond of comparatively favorable crystal quality has been obtained by optimizing the synthesizing conditions in a microwave-plasma CVD technique with phosphorous and sulfur as dopants. What is more, by combining n-type semiconductor diamond doped with the phosphorous and sulfur dopants, with p-type semiconductor diamond doped with boron, pn junctions are being formed to prototype UV-emitting LEDs.
Nevertheless, among single-crystal n-type semiconductor diamond of favorable crystal quality, even in the best-performing phosphorous-or sulfur-doped n-type semiconductor diamond, the resistivity at room temperature is at the 104 Ω·cm level, which is so high compared with the resistance of other semiconductor materials as to put the diamond into the insulator category. Moreover, given that the dependence of the resistivity of these n-type semiconductor diamonds on temperature is considerable because their activation energy is extraordinarily large—for the phosphorous-doped diamond it is approximately 0.6 eV, and for the sulfur-doped it is approximately 0.4 eV—across a wide temperature range, stable application of devices employing these n-type semiconductor diamonds has been problematic.
Furthermore, the covalent radius of the carbon atoms constituting diamond is 0.077 nm, whereas the covalent radius of phosphorous is 0.106 nm, and the covalent radius of sulfur is 0.102 nm. Inasmuch as the covalent radii of phosphorous and sulfur are appreciably large compared with the covalent radius of carbon, a problem in situations in which vapor synthesis is carried out while doping with phosphorous and sulfur has been that if diamond is vapor-deposited to a thickness on the order of 10 μm or more, cracks appear in the deposited diamond.
Other than phosphorous and sulfur, dopants with which n-type semiconductor characteristics have been experimentally verified include nitrogen, but with the activation energy of diamond that has been doped with nitrogen being approximately 1.7 eV, and the electrical resistivity at room temperature being 1010 Ω·cm or more, nitrogen-doped diamond is an insulator.
Meanwhile, it is known that diamond into which lithium is added will exhibit n-type semiconductor characteristics. For example, Japanese Unexamined Pat. App. Pub. Nos. H03-205398, H04-175295, and H11-54443 disclose techniques wherein by means of a hot-filament CVD method or various plasma CVD methods, with lithium or else water or a liquid organic compound—either of which contains a lithium compound—as a source material, or otherwise in which lithium or a lithium compound is vaporized and introduced into the deposition reactor, lithium is doped during the vapor deposition of diamond to produce low-resistivity n-type semiconductor diamond.
Nonetheless, problems with these methods have been that stable electrical properties cannot be achieved because the lithium moves around within the diamond, and that during the vapor deposition of diamond, what with the lithium bonding with hydrogen that is incorporated at the same time, the lithium is not electrically activated.
In another example, a technique for obtaining low-resistivity n-type semiconductor diamond by introducing lithium into the diamond lattice interstices is disclosed in Japanese Unexamined Pat. App. Pub. No. H07-106266. This technique is a method in which, with a nitrogenous compound of lithium as a source material, an ECR plasma is used to dope lithium into diamond without damaging its crystal quality. With this method, however, inasmuch as the ionic diameter of lithium (0.060 nm) and the ionic diameter of nitrogen (0.027 nm) are each smaller than the covalent diameter of carbon (0.077 nm), lithium and nitrogen both can enter into the diamond lattice interstices without damaging its crystalline structure. A large amount of lithium and nitrogen ends up being doped into the diamond lattice interstices, yet with this method the density at which the lithium and nitrogen are incorporated is not readily controlled. A further problem with the nitrogen has been that though it is doped into the lattice interstices, it does not become activated as an n-type dopant at all; the nitrogen doped in large quantity in the lattice interstices drastically deteriorates the n-type semiconductor characteristics such that the low-resistivity n-type semiconductor properties sought cannot be achieved.